When they got home, their uncle placed the two beetles in a glass tank by themselves, so that they could be more easily observed. “What do you call them, uncle?” asked Frank. “We have never seen such large, handsome beetles before.” [Illustration: Dyticus Beetle, male and female.] “Have you not?” his uncle replied. “I am surprised at that, for this beetle is found in nearly all our ponds and ditches. It has a long Latin name,[2] which means _Bordered Diver_, but it is commonly known as the ‘great water beetle.’ There are several kinds of these diving beetles. This is the largest. Can you give me an idea of their size?” And Uncle George handed Frank a small measuring rule which he carried in his pocket. Frank looked at the beetles, and then moved his thumb nail along the rule. “About an inch and a half in length,” he said. “That is about right,” said his uncle. “Are these two beetles exactly alike?” “They are both about the same shape and colour. One is slightly bigger than the other. They are both greenish-black above and brown below. There is a curious broad border of yellow all round the edge of their bodies,” said Tom. “But they are not both exactly alike,” said Frank, “I notice that one is smooth on the back, while the other’s back is all grooved.” “You are right, Frank,” said Uncle George. “The one with the grooved wing-cases is the female. The one with the smooth cases is the male. [Illustration: Fore Leg of Male, showing Sucker Organ.] “Notice the long legs they have for swimming, and how they seem to oar themselves through the water. Notice also that the male has a large flat disc upon each of his fore legs. These are suckers, by which he can cling to things.” “Why do they come up to the surface so often?” Frank asked. “They cannot live without air. They carry a supply of air under their wing-cases. They just come up to renew it from time to time. This is done by thrusting the end of their bodies out of the water as you see.” “One would think they ought to thrust their heads out to get air,” said Tom. “That seems more natural to us, because we breathe with our mouths,” said Uncle George. “These animals take in air with their tails. A great many pond insects breathe in this curious way.” “What do they feed upon?” Frank asked. “They are fierce, greedy creatures,” his uncle replied. “They attack and kill worms, tadpoles, and even small fishes.” Uncle George threw a small worm into the tank. One of the beetles seized it at once and began to devour it greedily. Presently the other beetle seized an end of the worm. The boys watched closely, and saw that the beetles’ jaws moved from side to side like a pair of pincers. Next morning, when Uncle George and the boys went in to see the beetles, they found only one in the tank. The other had escaped during the night. After a careful search they found it, at the far end of the room, dead.